Composite boards and panels

ABSTRACT

A building panel ( 1 ) including a water resistant core ( 5 ) including thermoplastic material  21  and a surface layer ( 4 ) including thermosetting resins. Also, production methods to form a board material ( 1 ′) with a dry blend of thermoplastic particles  21   a  in powder form and fillers in powder form and to apply a surface layer ( 4 ) with a hot-hot lamination process to a core ( 5 ) including such board material.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.14/672,610, filed on Mar. 30, 2015, which claims the benefit of SwedishApplication No. 1450382-5, filed on Mar. 31, 2014. The entire contentsof U.S. application Ser. No. 14/672,610 and Swedish Application No.1450382-5 are hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure generally relates to the field of building panels,preferably floor panels, comprising a mix of plastic materials in powderform, preferably thermoplastic material and thermosetting resins, andfillers in powder form, preferably wood based and mineral fillers.Production methods to form such panels comprising locking systems arealso disclosed.

FIELD OF APPLICATION

Embodiments of the present invention are particularly suitable for usein floors, which are formed of floor panels comprising a core, adecorative wear resistant surface layer and preferably a balancing layeron the rear side of the core. The following description of techniques,problems of known technology and objects and features of the disclosurewill therefore, as a non-restrictive example, be aimed above all at thisfield of application and in particular at floorings which are installedfloating with a mechanical locking system.

It should be emphasized that embodiments of the invention may be used toproduce building panels in general such as, for example, wall panels andfurniture components.

BACKGROUND

The following description is used to describe the background andproducts, materials and production methods that may comprise specificparts of preferred embodiments in the disclosure. The floor typesdescribed below typically have two major common features. They areintended to be installed in a floating manner with a mechanical lockingsystem and the decor is, in most cases, obtained by colour pigments.

The different flooring types described below use two major classes ofpolymers, thermosetting and thermoplastic polymers, to provide a bondingbetween particles, mainly wood and mineral fillers. Such fillers areused to improve mechanical properties or just as extenders to decreasethe amount of polymers used.

Thermosetting polymers are characterized in that they melt, float andpolymerize with increased stiffness when heat and pressure is applied.They are completely cured and cannot be melted again. Thermoplasticparticles on the other hand soften and melt when temperature isincreased and the stiffness increases when they are cooled down.

Thermoplastic polymers are very moisture stable and may be formed as aproduct comprising only a specific polymer material. Thermosettingpolymers are moisture sensitive and very brittle. They must be combinedand reinforced with fibres, generally with wood fibres.

1. Thermosetting Flooring

This flooring category is characterized in that thermosetting resinssuch as, for example, Melamine Formaldehyde (MF), Urea Formaldehyde (UF)or Phenol Formaldehyde (PF) resins are combined with mainly wood fibresand cured under heat and pressure such that separate core or surfacelayers may be formed.

a) Laminate Floors

The majority of all laminate floors have a wood design. Such laminatedfloors have a core of 6-12 mm fibreboard, a 0.2 mm thick upperdecorative surface layer of laminate and a 0.1-0.2 mm thick lowerbalancing layer. The decorative and wear properties are generallyobtained with two separate layers of melamine formaldehyde impregnatedpaper (a decorative layer and a wear layer), one above the other. Thedecorative layer is a printed paper and the wear layer is a transparentoverlay paper, which comprises small aluminium oxide particles. The coreis generally a high-density fibreboard (HDF) comprising wood fibres anda mix of MF and UF resins. The decorative paper and the overlay paperare directly laminated by a hot-hot production method to the HDF coreunder high heat and pressure (170° C., 40 bar). Hot-hot pressinggenerally refers to a pressing method where a press table hasessentially the same closing temperature as the opening temperature(e.g., within about 10° C.). Hot-hot pressing is very cost efficientsince no cooling of the press table is needed but can only be used indirect lamination of thermosetting based surfaces that cure at high heatand that are laminated to rather thick core materials comprisingthermosetting resins that are able to absorb the steam from the moistureof the core and the surface layer that is created at high pressingtemperatures above 100° C.

A separate laminate sheet may also be produced in a continuous ordiscontinuous pressing operation and the laminate sheet is thereafterglued to a core material. This gluing, that may be made in coldcondition, allows that separate laminate sheets may be bonded todifferent core materials, provided that they have similar moisturemovements as the surface layer when relative humidity (RH) variesbetween dry (RH 20%) and wet conditions (RH 90%).

b) Powder-Based Floors

Recently new “paper free” floor types have been developed with solidsurfaces comprising a substantially homogenous powder mix of fibres,binders and wear resistant particles hereafter referred to as WFF (WoodFibre Floor).

The powder mix may comprise aluminium oxide particles, thermosettingresin such as melamine formaldehyde resins and wood fibres. In mostapplications decorative particles such as, for example, colour pigmentsare included in the mix. In general, all these materials are applied indry form as a mixed powder on a wood-based core such as a HDF core andcured in a hot-hot direct lamination step under similar heat andpressure as used in laminate floors.

Digital powder printing has been developed and it is possible to createvery advanced wood, stone and tile designs by injecting water based inkdrops comprising pigment dispersions into the powder prior to pressing.A recently developed Binder and Powder (BAP) printing method may also beused to provide a digital print on a powder-based surface. Conventionaldigital printing methods are divided in two separate steps comprising aseparate application of a liquid binder, generally referred to as “blankink” and of dry colorants, generally referred to as “dry ink”. Colouredparticles of dry ink are applied on a panel surface. Some particles arebonded by a digitally formed pattern of blank ink applied by a printhead. Other non-bonded dry ink particles are removed and the remainingbonded particles form a digital decor. This two-step process may berepeated and several colours of dry ink may be applied such that anadvanced multi colour high definition digital print may be formed in acost efficient way. The major advantage is that rather simple waterbased liquid substances may be used and the pigment based dry inkcolorants are very cost efficient since no pigment dispersions areneeded.

2. Thermoplastic Flooring

This flooring category is characterized in that thermoplastic materialssuch as, for example, polyvinyl chloride (PVC), polypropylene (PP), orpolyethylene (PE) are mixed with mineral or wood fibres. The finalpressing is made under lower pressure than generally used in, forexample, laminate production and with a hot-cold pressing method. Thetemperature is generally similar, about 140-160° C. Thermoplasticmaterial melts at high temperatures and no curing takes place. The presstable or belt must therefore be cooled down below 100° C. such that thethermoplastic layer is converted from a floating paste to a solid layer.Separate thermoplastic layers may be fused or laminated together byheating and cooling under pressure.

c) LVT Floors

Luxury Vinyl Tiles, generally referred to as LVT floorings, aregenerally constructed as a layered product made of thermoplasticmaterial such as PVC, generally referred to as vinyl, mixed with mineralfillers, colour pigments and plasticizers. The name LVT is somewhatmisleading since a major part of LVT floors have a plank size with awood design. They may also comprise other plastic materials that are notvinyl. LVT has become a generic name for high quality resilient floorthat may be installed floating with a locking system.

Thermal moulding is used to form the PVC layers. Co-rotating twin-screwextruders are used for the production of high-quality thermoforming PVClayers with fillers. The screws press and mix the material under heat.

The extruder may be filled with pre-formed small plastic pelletscomprising a compound of PVC, pigments, fillers etc. Compounding is aprocess where mixing of polymers and additives in a molten state underheat and generally also under pressure is used in order to form plasticpellets comprising a homogenous blend of the different raw materials.Co-kneaders, twin-screw extruders and internal mixers are the mostcommon compounders used in the plastic industry.

The compounding process may be avoided and an extruder may also bedirectly filled with basic plastic particles and separate fillers andother additives that are mixed in a hopper attached to the extruder.This method may be used to save material cost. Advanced twin-screwextruders with optimized screw geometry combined with accuratetemperature control are used to obtain high melt homogeneity.

The extruder comprises a flat sheet die that provides a paste withessentially the same width as the finished layer. The die has generallyoil temperature-controlled adjustable die lips that provide a widethickness range.

The melted paste is further processed with 3-roll polishing calenderscomprising a fixed central hot roll and two adjustable rolls on eachside for calibration. The first roll is hot and the third roll is cold.The melt feed angle, which is determined by the weight and elongation ofthe melt, can be changed by adjusting the position of the first roll.Horizontal, inclined or vertical roll arrangements may be used.

A wide range of downstream equipment and methods are used to laminatethe layers under heat and pressure and to cool the plastic layers to amulti-layer sheet. One simple method is to cut the layers to sheets andto use a multi opening hot-cold press. In continuous production severalextruders may be used and the layers are hot when they are laminatedtogether, in most cases with an intermediate layer of glass fibre.Cooling and stabilization of the panel in order to avoid warping takes alot of space and a continuous production line may be several hundredmeters long.

Extruders and calenders may be replaced with a continuous double beltpresses comprising heating and cooling under pressure combined withseveral nip rollers. Plastic compounds in pellet form are scattered onthe lower belt and pressed together under heat and pressure. A similarcontinuous press may also be used to laminate several sheets and foilsto a LVT floor panel.

The core is made primarily of several layers that comprise PVC mixedwith chalk and/or limestone fillers. The most common core materialscomprise a middle part with about 50% limestone and a balancing layerwith about 40% limestone. The lower limestone content in the backing isused to balance the high plastic content in the top layer. Some LVT corematerial may comprise up to 80% limestone fillers. Such core materialswith high filler content are very brittle and it is difficult to form astrong locking system.

A 0.1 mm high quality printed decorative PVC foil is applied on theupper side of the core. A transparent wear layer of vinyl with athickness of 0.1-0.6 mm is generally applied on the decorative foil. Thebase layer, the decorative foil and the transparent layer are fused orlaminated together with heat and pressure in continuous or discontinuouspress operations to a 3-6 mm floor panel. A transparent UV curingpolyurethane (PU) lacquer is generally applied as a protective layer onthe transparent PVC foil. LVT floor are installed floating with alocking system similar to laminate and powder based floors.

The decorative effects are obtained with a decorative foil that isprinted separately and fused on the core layers. The foil comprises asingle colour that generally is white and covers the dark colour of thecore. The foil provides a base colour for the rotogravure printingprocess where special solvent-based inks comprising pigments are used tocreate wood, tile and stone designs.

d) WPC Floors

Wood Plastic Composite (WPC) panels comprising thermoplastic materialmixed with wood particles have been used in extruded sections andinjection mouldings in several industries during many years. RecentlyWPC flooring panels in plank sizes have been developed for outdoor usemainly as decking. Thermoplastic material such as PVC or PP is mixedwith wood in the form of sawdust or wood chips by wood particle weight(wt) of about 50% to 80%. Products intended for exterior use maycomprise 50 wt % or less of wood particles. The wood content may exceed70 wt % in products intended for interior use. The processing is similarto the methods used to produce a LVT floor. A two-step process may beused where wood fibre particles and plastic particles in a first stepare compounded together under heat and pressure to plastic pellets andfed into an extruder. Fibres and plastic granules may also be mixeddirectly during extrusion. Alternatively compounded plastic pellets maybe scattered on a belt and pressed to WPC sheets in a continuous doublebelt press comprises a heating and a cooling section as described above.The decorative properties are mainly obtained by pigments that areincluded in the wood/plastic mix. WPC panels may also be printed,painted or foiled.

3. Combi-Floor

Thermoplastic and thermosetting materials may be combined. ThermoplasticLVT layers may, for example, be glued to a HDF core or a WPC core.

Definition of Some Terms

In the following text, the visible surface of the installed floor panelis called “front side or upper side”, while the opposite side of thefloor panel, facing the sub floor, is called “rear side or lower side”.

By “up” is meant towards the front side and by “down” towards the rearside. By “vertically” is meant perpendicular to the surface and by“horizontally” parallel to the surface.

By “panel” is meant an essentially flat sheet that comprises a core anda decorative surface formed on the core.

By “board” is meant an essentially flat sheet formed material that isintended to be covered by a surface layer and used as a core in a panel.

Known Technique and Problems Thereof

The general technologies, which have been developed by the industryaiming to improve the properties and cost structures of theabove-described floors are described below. The methods may be usedpartly or completely in various combinations with preferred embodimentsof the invention in order to create a board and a panel according tothis disclosure of the invention.

The above described floor types have different properties and coststructures. Laminate and WFF floors have superior surface propertiesrelated to wear, impact and stain resistance. The hot-hot directlamination method is very cost efficient. The major disadvantages arethat they are hard, moisture sensitive and give a high sound level whenpeople walk on the floor.

LVT floors on the other hand are soft, flexible, moisture proof andsilent. However, several properties are inferior to laminate floors. Theplastic surface is stain and heat sensitive and the production costs aregenerally higher than for laminate flooring since hot-cold pressingmethods must be used. An LVT floor panel expands 3 mm/m when thetemperature increases from 5-65° C.

The major advantage of WPC floors compared to LVT floors is that thewood fibres in the WPC floor counteract thermal expansion. WPC floor arealso more rigid and may overcome defects in the subfloor. However, WPCfloors are expensive to produce and it is difficult to apply anattractive decor layer that is wear and stain resistance and that has anappropriate moisture movement such that warping may be avoided. WPCfloor are harder than LVT floors and the sound level is generallyhigher.

In general it is easy to form a strong locking system in a HDF core. WPCpanels are rather rigid and the locking system may have similar strengthor even higher compared to HDF based laminate floors. To form lockingsystems in a LVT floor is much more complicated due to the fact that thecore is thinner and softer. A disadvantage is the fact that the lockingstrength of a LVT based locking system decreases considerably atincreased temperatures when the core becomes softer.

It would be an advantage if a water proof thermoplastic core withsufficient strength to form a mechanical locking system and favourablemoisture and thermal properties may be produced in a more cost efficientway and if such a core may be combined with a high quality surface layercomprising thermosetting resins, such as melamine formaldehyde resins.Considerable advantages may be obtained if such surface layer may bedirectly laminated with a cost efficient hot-hot pressing method to athermoplastic core.

One problem that is preferably solved is the shrinking of the melamineresin that takes place during pressing when the resin cures and afterpressing when the panel is cooled down to room temperature. Suchproblems have been solved when direct lamination is made against a woodbased panel such as HDF and particleboard comprising thermosettingresins. The problems are not solved when lamination is made against acore comprising thermoplastic materials.

Another problem that is preferably solved is the different humidityvariations of thermosetting and thermoplastic materials. As an exampleit may be mentioned that a surface layer comprising wood fibres andthermosetting melamine resins may expand about 0.8% when the relativehumidity increases from 20 to 90%. Comparable expansion of a HDF corebased on thermosetting melamine/urea binders may be about 0.4% and sucha difference is possible to overcome with a strong lamination betweenthe wood fibres in the core and the thermosetting resin in the surfacelayer. The lower moisture movement of the core will counteract thehigher moisture movement of the thermosetting based surface layer and alaminated panel may have a moisture movement which is an average of thetwo movements, for example, a moisture movement of about 0.6%.

The humidity variations of a WPC panel may be only 0.05% during a timeperiod when a melamine formaldehyde layer expands 0.8%. LVT panels areeven more humidity stable and practically no expansion may be measured.Different humidity movements between a humidity sensitive surface layercomprising, for example, melamine laminated to a humidity stable WPC orLVT core comprising thermoplastic material will create large shearforces between the surface layer and the core and this may causedelamination. Furthermore it is also very difficult and in factpractically impossible to laminate a thermosetting layer with a hot-hotdirect lamination method to a conventional WPC or LVT layer where allfillers are embedded in a thermoplastic layer. Conventional digital inkjet printing and digital BAP printing as described above are based onthe main principle that the ink drops should not float or form clusterswhen they hit the surface. It is difficult to use digital printing ondense and non-absorbing surfaces such as thermoplastic surfaces. Itwould be a major advantage if surfaces comprising thermoplastic resinsmay be formed such that floating of ink drops may be avoided.

WO 96/27721 describes a WPC type of floor panel with a core of woodparticles impregnated with a thermoplastic and with a decorativethermosetting laminate as a surface layer glued on top of the core. Itis difficult to glue a thermosetting material to a thermoplastic WPCcore and the disclosure does not give any description of how the gluingshould be made. The materials have different humidity and moisturemovements and such panels will warp or the glue layer will break whenrelative humidity varies between 20 and 90%.

EP 1847385 B1 describes a construction board with a core made of woodchips or wood fibres, which are glue-treated and pressed together, andtwo cover layers at the upper side and the lower side consisting ofscattered WPC material. The core and the WPC cover layers are pressedtogether under heat such that the thermoplastic plastics materials ofthe WPC layers are bonded to the wood core. It is mentioned that thesurface can be provided with a variety of building board coatings but nospecific surface layer is disclosed. The bending strength may beincreased and humidity movements may be reduced but a conventional woodbased core layer in the middle parts such as particle board, HDF or OSBis not water proof and will swell considerably if exposed to water thatmay penetrate into the joints between adjacent floor panels.

WO 2008/122668 A1 describes a method of manufacturing laminate floorpanels comprising a WPC core, a basic layer of paper or glass fibremelted to the WPC core and a top laminate comprising at least one paperlayer impregnated with melamine resin, and being attached to the basiclayer through this impregnation material or a plastic layer melted to aglass fibre basic layer. Such a basic layer may provide a laminationbetween the core and the laminate surface. However the thermosettinglaminate layer will shrink and swell more than the WPC core and thepanels will warp at low and high relative humidity, the surface or theglue layer may crack or delaminate.

WO 2012/053036 A1 describes a flooring comprising a base material layerof LVT and a single sheet of WPC laminated on top of the LVT. Anadhesive layer is provided between the LVT and the WPC layer and anonwoven cloth is embedded into the adhesive layer.

A disadvantage is that the lower part of the panel comprises a soft LVTlayer and this part must be used to form the locking system. The lockingstrength will be low especially when the temperature increases due todirect sun exposure during the summer months or when floor heating isused.

EP 2402155 A1 shows a floor panel with a WPC layer and a reinforcementlayer. The reinforcement layer is intended to minimize dimensionalchanges of the WPC layer. The reinforcement layer is incorporated withinthe WPC layer such that portions of the WPC layer extend at both sidesof the reinforcement layer. The floor panel includes a high pressurelaminate construction comprising at least a printed decorative layer andan impregnated protective overlay pressed together with heat andpressure. Alternatively the WPC layer may be provided with a polymericfilm having a decoration pattern, which is printed on the film. Thepolymeric film may be melted to the WPC layer. Other alternatives arewood veneer layers and vulcanized cellulose layers.

The reinforcement layer may increase the possibilities to bond athermosetting laminate to a thermoplastic core but it will not eliminatethe problems related to different moisture movements of a thermosettingtop layer connected to a thermoplastic core.

WO 2009/065769 (Välinge) describes that a panel may be formed by apowder mix of wood fibres and binders. Thermosetting binders may be usedin the core and thermoplastic binders in the surface layer or theopposite.

WO 2011/129757 (Välinge) describes a digitally printed surface layerthat may comprise a thermoplastic material, for example vinyl, appliedin powder form on a substrate. Powder layers may comprise a mix of woodfibres, thermosetting binders in powder form and thermoplastic powderparticles. It is also described that a powder based surface layercomprising thermoplastic particles may be applied on a core, preferablyof plastic or wood, which may comprise one or several layers withdifferent density. A LVT floor panel can be produced with a powder basedand digitally printed surface layer.

US 2003/0008130 describes a water resistant particleboard comprisingwood particles, thermosetting resins and resin particles of athermoplastic resin. The particleboard comprises thermoplastic sheetmaterial at the upper and lower side. The wood and thermoplasticparticles have an average particle diameter of 1-2 mm. The averageparticle size of the outer layers is smaller than the particle size ofthe middle layer.

The three applications above do not provide solutions to bonding andmoisture movements of a surface layer comprising thermosetting binderslaminated to a core comprising thermoplastic material.

As a summary it may be mentioned that none of the above describedmethods may be used to obtain a considerable improvement of the surfaceproperties, stability and cost structures of a thermoplastic based floorintended to be installed floating with a strong locking system.

Objects and Summary

A general objective of at least certain embodiments of the invention isto provide a thermoplastic based building panel that has better surfaceproperties and more favourable cost structure that known building paneltypes.

A first specific objective of at least certain embodiments of theinvention is to provide a panel, preferably a building panel, with acore comprising thermoplastic material, which gives the panel improvedwater resistant properties, and a surface comprising thermosettingresins that give the panel improved surface properties. The core and thesurface should be such that a strong bonding may be obtained with heatand pressure and such that humidity movements of the materials in thecore and in the surface are adapted to reduce warping and large shearingforces.

A second specific objective of at least certain embodiments of theinvention is to form a board material that may be used as a core in suchpanels.

A third specific objective of at least certain embodiments of theinvention is to adapt the core such that a strong locking system may beformed in such floor panels.

Embodiments of the invention are based on an understanding thatconventional wood plastic composite materials such as WPC are mainlydesigned for construction purposes where high loads must be handled fora long time in outdoors environment. Performance optimising has beendirected towards high bonding strengths between the wood fibres andplastic material, UV stability, bending strengths, deformation underload and similar. Several additives are included such as coupling agentsthat increase the bonding between the wood particles and thethermoplastic material and UV stabilizers that counteract colourchanges. A mix of wood and thermoplastic particles is melted underpressure to form compounds of wood/plastic pellets that provide ahomogeneous high quality panel after pressing or extrusion.

Such optimising is not suitable for floating floor applications wherethe panel is laying on a supporting sub floor in indoor environment andis covered by a protective surface layer. In fact it provides anunnecessarily high cost structure and the material properties can causemajor problems related to the possibility of applying a surface layercomprising thermosetting resins especially if direct lamination is usedbased on a hot-hot pressing step. No attempts have been made to form aboard that is adapted to provide a strong locking system.

Embodiments of the invention are based on a second understanding thatthe material properties of the core and preferably also of the surfacelayer must be changed compared to conventional technology in order toform a panel that has all the favourable core properties ofthermoplastic based floor types but also the favourable surfaceproperties of laminate and wood powder based floors where the upperlayer comprises a high content of melamine formaldehyde resins. Ingeneral terms it means that a core comprising thermoplastic materialshould be adjusted such that it performs more in a way similar to alayer formed by thermosetting resins. It is preferred that the surfacelayer comprising thermosetting resins is adjusted such that is behavesmore as a thermoplastic material.

According to a first aspect of the invention a method to produce abuilding panel is provided. The method comprises the steps of:

-   -   providing a core by forming a mat shaped layer comprising a dry        blend of wood particles in powder form and thermoplastic        particles in powder form, heating and cooling the mat shaped        layer under pressure such that a sheet is formed after pressing;    -   applying a surface layer comprising a powder mix comprising wood        particles and thermosetting resin on the core, and;    -   laminating the powder based surface layer in a hot-hot pressing        operation to the core and thereby forming a building panel,        wherein the wood particles of the surface layer and of the core        are bonded to each other by cured thermosetting resin.

The sheet formed after pressing may be flat.

The surface layer may further comprise a thermoplastic material. Thethermoplastic material may be located between the bonded wood particles.The thermoplastic material may comprise thermoplastic particles. Inparticular, the thermoplastic material may comprise melted thermoplasticparticles. The thermoplastic particles may be comprised in the powdermix.

Forming the board from a dry blend and mixing thermosetting particlesinto the surface layer will increase the moisture movement of the core,decrease the moisture movement of the surface layer and improve thebonding properties such that a direct lamination in a hot-hot pressingoperation may be made of the surface layer to the core.

The thermosetting resin may be an amino resin such as melamineformaldehyde resin, urea formaldehyde resin, a phenol formaldehyderesin, or a combination thereof.

The thermoplastic particles of the dry blend may comprise polyvinylchloride (PVC), polypropylene (PP) or polyethylene (PE).

The thermoplastic particles of the dry blend may have an averageparticle size of about 0.3 mm or less.

The surface layer may be applied as two layers arranged above each otherand an upper layer of the two layers may comprise bleached fibres andthermosetting particles. The bleached fibres may be bleached cellulosefibres.

The wood particles of the core may prior to application be dried to amoisture content of about 2% or less.

The wood particles of the core may prior to application be sieved to anaverage size of 1.0 mm or less.

The content of the wood particles in the core may be in the range ofabout 30-80 wt %.

The core may be formed with a pressure which is lower than 20 bar.

The thermoplastic material may be comprised in the powder mix of thesurface layer and may comprise thermoplastic particles. Moreover, thethermoplastic particles may have an average size of about 0.2 mm orless.

The wood particles of the surface layer may be smaller than the woodparticles of the core.

The core may comprise an intermediate layer and an upper layer. Theintermediate layer may comprise a higher amount of thermoplasticmaterial than the upper layer.

The intermediate layer may further comprise limestone particles.

The surface layer may comprise a transparent wear layer comprisingthermosetting resins, wear resistant particles, such as aluminium oxideparticles, and cellulose fibres, preferably bleached cellulose fibres.

The wood fibers of the core and/or the surface layer may be cellulosefibres, such as bleached cellulose fibres.

The thermosetting resin of the transparent wear layer may be an aminoresin such as a melamine formaldehyde resin, a urea formaldehyde resin,a phenol formaldehyde resin, or a combination thereof.

According to a second aspect of the invention a building panelcomprising a core and a surface layer is provided. The core comprises amix of thermoplastic material and wood particles. The surface layercomprises a composite material comprising wood particles bonded to eachother by a cured thermosetting resin. The surface layer is bonded to thecore by the cured thermosetting resin.

The surface layer may comprise a composite material comprisingindividual thermoplastic particles spaced from each other and surroundedby wood particles bonded to each other by a cured thermosetting resin.

The surface layer may be bonded to the core by fused thermoplasticparticles and cured thermosetting resins.

The thermosetting resin may be an amino resin such as a melamineformaldehyde resin, a urea formaldehyde resin, a phenol formaldehyderesin, or a combination thereof.

The thermoplastic material of the core may comprise PVC, PP or PE.

The surface layer may comprise thermoplastic particles with a size ofabout 0.2 mm or less.

The surface layer may comprise wood particles with an average size thatis smaller than the average size of the wood particles of the core.

The building panel may be a floor panel.

One pair of opposite edges may be formed with a mechanical lockingsystem comprising a protruding strip. Moreover, the protruding strip maycomprise wood particles and thermoplastic material.

According to a third aspect of the invention a building panel having acore, a surface layer and a two opposite edges comprising a mechanicallocking system is provided. The building panel comprises a tongue andgroove for vertical locking and a protruding strip with a lockingelement in one edge cooperating with a locking groove in another edgefor horizontal locking. The core is formed with three layers ofmaterials. Each layer comprises thermoplastic materials and fillers. Anupper and a lower layer comprise fillers of wood particles and anintermediate layer comprises fillers of mineral particles.

The locking element may be formed in the lower layer. Moreover, theprotruding strip and the locking element may be formed in the lowerlayer. Additionally, the protruding strip, the locking element and thelocking groove may be formed in the lower layer.

The locking element and the locking groove may be formed in the lowerlayer.

The building panel may be a floor panel.

The building panel may comprise a surface layer comprising woodparticles and thermosetting resins.

The principles of embodiments of the invention may be used to form aboard that may be used as a core in building panels, especially floorpanels that comprise a surface layer with thermosetting resins that isdirectly laminated to the core. The board surface is adapted to suchlamination since a considerable amount of free wood fibres are exposedat the surface and may be bonded with cured thermoplastic resins fromthe surface layer.

According to a fourth aspect of the invention a board is provided. Theboard comprises an upper surface, a lower surface, and an intermediateportion located between the upper surface and the lower surface. Theintermediate portion comprises wood particles embedded in athermoplastic material. The upper surface comprises essentially raw woodparticles adapted to bond to a thermosetting resin.

At the upper surface, the essentially raw wood particles may protrudebeyond the upper surface.

The thermoplastic material may comprise PVC, PP or PE.

The average size of the wood particles in the upper surface may be about0.1-1.0 mm.

At the lower surface, wood particles may protrude beyond the lowersurface.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will in the following be described inconnection to embodiments and in greater detail with reference to theappended exemplary drawings, wherein,

FIGS. 1a-d Illustrate a board and a panel with a core of such boardmaterial, a surface layer and a balancing layer,

FIGS. 2a-h Illustrate board material produced by dry blends andthermoplastic compounds,

FIGS. 3a-d Illustrate production methods to form boards and panels,

FIGS. 4a-4d Illustrate floor panels with a locking system.

DETAILED DESCRIPTION

FIG. 1a shows a cross section of a board 1′ according to a preferredembodiment of the invention. The board 1′ is formed by a dry blendcomprising a mix of thermoplastic particles in powder form and woodparticles 20 in powder form which is pressed together under hightemperature. The thermoplastic particles melt and form a thermoplasticmaterial 21 that bonds the wood particles 20. With dry blend is meant apreferably uniform product comprising a dry mix of at least twodifferent and separate materials in particle form that are preferablyrandomly distributed in the mix. The individual materials may segregate,for example due to vibration or material flow when forces are acting onthe individual particles.

The dry blend is preferably applied as a dry powder layer on a lowerbelt of a continuous press, heated and cooled under pressure. The powderis pressed as a dry blend and not as a pre-formed plastic compound inpellet form. With plastic compound is meant a preferably uniform productcomprising a mix of materials wherein at least one of the materials isin a molten state during mixing. The individual materials in a plasticcompound particle cannot separate after mixing due to, for example,vibration.

Pressing parameters such as pressure, pressing time, pressing andcooling temperatures are adapted such that the production method mayprovide a board material that compared to a conventional WPC board issufficiently water proof such that it may be used as a core in a waterresistant floor panel but may have a higher humidity movement and maycomprise wood particles 20 in the upper part 11 and the lower part 12that are not completely covered by a plastic material 21 and that may bebonded to other fibres with a thermosetting resin when a fibre basedsurface layer is applied.

A board 1′ may be formed with an intermediate portion 13 comprising woodparticles 20 embedded in a thermoplastic material 21 and with an upperpart 11 preferably comprising raw wood particles 20 that are essentiallynot covered by melted plastic particles and that may protrude slightlyabove the thermoplastic material 21 after pressing. With raw woodparticles is meant a wood particle having at least one surface portioncomprising wood material that is not covered by another material, forexample a thermoplastic material. Preferably the lower part 12 comprisessimilar raw wood particles 20 protruding below the thermoplasticmaterial 21. The raw wood particles 20 in the upper 11 and lower parts12 are adapted such that they may be used to facilitate a strong bondingbetween the core and separate layers comprising, for example, athermosetting resin such as a melamine formaldehyde resin that duringlamination with heat and pressure may penetrate into the raw woodparticles. The board 1′ may be formed with a porosity that provides anappropriate humidity movement adapted to the humidity movement of asurface layer comprising, for example, a thermosetting resin.

As a non limiting example it may be mentioned that the dry blend may bepre-heated to about 80-120° C., e.g., about 100° C. prior to pressing,pressed with about 5-15 bar, e.g., about 10 bar in a heating zone undera temperature of about 130-200° C., e.g., about 160° C. and cooled to50-100° C., e.g., about 80° C. under pressure in a cooling zone. Thepressing time in the heating zone may be about 1-3 minutes, e.g., about2 minutes, for a board that is about 4-5 mm thick.

The dry blend may, for example, comprise 40-60% by weight (wt %), e.g.,48 wt % PVC powder (e.g. Vestolit 6507) with particle size of about0.08-0.2 mm, e.g., 0.1 mm, 40-60 wt %, e.g., 48 wt %, wood particles inpowder form with an average length of about 0.5-2 mm, e.g., about 1 mm,and preferably 1-10 wt %, e.g., 4 wt %, colour pigments. A Wood PlasticParticle board hereafter referred to as a WPP board may be formed in acost efficient way.

The WPP board is preferably 3-12 mm thick and the scattered dry blend ispreferably compressed to about 25-30% of its original thickness to arigid board with a preferred density of about 1100-1300 kg/m3. An evenmore preferred thickness range for a board material intended to be usedas a core for floating floor panels in home environment is about 3-6 mmand such a board is preferably formed from a dry blend having athickness after scattering but prior to pressing of about 1-2 cm.

Several thermoplastic materials that are available in powder form may beused such as, for example, polyvinyl chloride (PVC), polypropylene (PP)and polyethylene (PE) and they may be mixed. The moisture content of thewood particles is preferably 1-2% but moisture contents preferably up toabout 6% may also be used. The wood particles may comprise individualfibres or several fibres connected to each other. The particle thicknessis preferably smaller than the particle length. The average length L ofthe wood particles is preferably 0.1-1.0 mm. The particles may alsocomprise cellulose fibres that may be bleached cellulose fibres.

The dry blend is preferably scattered on a belt as a mat shaped layerand scattering forms the required thickness. The dry blend may also beapplied as a thick layer that is equalized to required mat thicknesswith a ruler that removes excess material.

It is preferred to use wood particles with low moisture content,preferably 2% or lower, especially when the WPP panel in a secondproduction step is used as a core that is direct laminated in a hot—hotpressing operation with a layer comprising thermosetting resins. Highmoisture content creates steam that may cause surface defects such aslocally delaminated surface portions.

An optimization of wood particle size and content, pressure, pressingtime and temperature may be used to form a board with desiredproperties. For example, small well defined particles with an averagesize of less than 1.0 mm, a high resin content >50 wt %, high pressureof about 40-60 bar, longer pressing time and high pressing temperatureup to 180-190° C. will improve water resistance and climate stability ofa WPP panel. The disadvantages are that density and material cost willincrease and it will be more difficult to bond a surface layercomprising thermosetting resins to the WPP panel. Consequently, thereare some limitations that may be considered when the objective is toform a cost efficient panel with appropriate properties that is possibleto combine with surface layers comprising thermosetting resins.

Floating of thermoplastic particles increases when melting temperatureis increased from, for example, 160 to 200° C. Wood particles willchange colour at increased temperature above 190° C. and the propertieswill deteriorate considerably above 200° C. A high pressure, forexample, 30-40 bar and even a pressure of 60 bar and higher combinedwith a high temperature, for example, 190° C. will provide a veryhomogenous high density and moisture stable material with highmechanical strengths. Such board materials may, for example, be suitablefor construction purposes, outdoor use and for floors with very highwater resistance but may generally not be the most preferred solutionswhen the board is intended to be used as a core in a moisture resistantfloating floor for indoor use with a moisture sensitive surface layercomprising thermosetting resins. Therefore, in some applications, ratherlow pressures around 5-20 bar and pressing temperatures of 160-180° C.may be preferred since this will reduce the density and the floating ofthe melted thermoplastic particles. In some applications it is alsopossible to produce the board with a low pressure of a few bar only.

A panel with a porous microstructure may be formed with properties thatprovide a suitable moisture movement and a base for direct lamination ofa surface comprising thermosetting resins.

The pressing time in relation to board thickness is also important. Longpressing time will create more floating than a shorter pressing time.Thicker boards require a rather long pressing time until an appropriatetemperature is reached in the middle section of the board. A longpressing time may damage the plastic material in the upper and lowerparts close to the press tables that are exposed to higher temperaturesduring a longer time than the middle section. A thicker board of, forexample, 6 mm may be produced as two 3 mm boards that are fused togetherwith heat and pressure.

A high starting temperature may reduce the pressing time considerably.For example, the pressing time of a 4 mm board under 160° C. may bereduced from 4 minutes to 2 minutes if the dry blend is pre-heated to100° C. prior to pressing. A preferred pre-heating is 80-120° C. Ofcourse higher temperatures up to, for example, 150° C. may be used. Thedry blend is preferably pre-pressed prior to the pre-heating and/or thefinal pressing such that a major part of the air between the particlesis removed.

A WPP board is preferably prior to the pressing step formed with severallayers comprising different material compositions. The middle part ofthe core 5 that preferably comprises about 60% or more of the corethickness may, for example be formed from a dry blend comprising 40-60wt %, e.g., 50 wt %, wood particles and 40-60 wt %, e.g., 50 wt %,thermoplastic particles. This may provide a high water resistance. Thedry blend in the upper and lower portions of the core 5 may comprise,for example, 50-70 wt %, e.g., 60 wt %, wood particles and 30-50 wt %,e.g., 40 wt %, thermoplastic particles and this may provide a highbonding strength between the core 6 and a surface layer 4 comprisingthermosetting resins.

The thermoplastic particles of the dry blend melt during pressing andform a plastic material that at least partly surrounds the woodparticles. In particular, the plastic material may completely surroundthe wood particles. All parameters disclosed above are preferablyadapted to the type and size of thermoplastic particles that are usedsince they may have different melting temperatures and floatingproperties.

A WPP board may have very favourable properties. For example, a boardbased on 40-60 wt %, e.g., 50%, PVC powder and 40-60 wt %, e.g., 50%,virgin wood powder of soft wood such as, for example, spruce or pine iswaterproof and thickness swelling after 24 hours in water may be about3-5%, e.g., about 4%. This could be compared to conventional HDF thatswells 16% and WPC that swells 2%.

The moisture movement along the board, when the relative humidity RHchanges from 20% to 90%, may be about 0.1-0.2% compared to 0.4% for HDFand 0.05% and lower for WPC.

The wood particle content may vary preferably between 20-80 wt % andeven more preferred between 40-60 wt %. Small wood particles sievedtrough a mesh of 0.3 mm and small thermoplastic particles sieved trougha mesh of 100 microns provide after pressing a more homogenous boardthan, for example, larger wood or thermoplastic particles in the rangeof 0.6 mm for wood particles and 200 microns for thermoplasticparticles. This may be used together with the pressing parametersdescribed above to optimize the moisture movements and the bondingproperties of the board to various other layers applied on the upper 11and/or the lower 12 part.

The whole board 1′ may be scattered in one operation. The board may alsobe scattered as a multilayer board where several layer are applied oneach other with preferably different material compositions. The size andthe material properties of the particles and the mix betweenthermoplastic particles and wood particles may vary in the layers.

The board may be used as a floor panel without any additional decorativesurface layers. Pigments may be sufficient to create a decor. In someapplications only a protective lacquer may be sufficient. Wear resistantparticles may be included in the upper part 11 of the board. Prints,preferably digital prints, applied directly on the WPP board or on anintermediate plastic foil preferably covered by a protective lacquer,such as, for example, a UV curing lacquer and/or a thermoplastictransparent foils may also be used.

An advantage is that the properties of the fibre structure in the upperpart 11 form a print layer such that a digital print, preferably a waterbased digital ink jet print comprising pigment dispersions, may beapplied with reduced or even without ink drop bleeding that generally isthe case when water based ink drops are applied on a thermoplastic nonabsorbing surface.

The Binder and Powder (BAP) printing method may also be used to providea digital print on the board. A liquid binder, generally referred to as“blank ink”, comprising for example water and viscosity increasingsubstances, such as for example glycol, is applied by a Piezo print headin a pre-defined pattern on the upper part 11 of the core 5. Colorants,generally referred to as “dry ink”, comprising for example wood fibresand colour pigments are applied on the liquid pattern. Some colorantsare bonded by the digitally formed liquid pattern of blank ink. Othernon-bonded dry ink colorants are removed and the remaining bondedcolorants form a digital decor. This two-step process may be repeatedand several colours of dry ink may be applied such that an advancedmulti colour high definition digital print may be formed in a costefficient way. The major advantage is that the wood fibres in the upperpart of the board reduces or eliminates uncontrolled floating of theblank ink drops and a high quality BAP print may be obtained on a boardthat comprises non absorbing thermoplastic material.

In some applications it is preferred to use bleached cellulose fibres inthe upper part 11 of the core 5. Such fibres may increase the printingquality when conventional direct printing or digital printingtechnologies are used. The bleached cellulose fibres may comprise colourpigments that may be used to form a base colour for the print.

The wood particles may be combined with other fillers, for example,mineral fillers such as stone powder, quartzous sand, clay materials,feldspathic fluxes, chalk, limestone and similar. Plasticizers andfoaming agents may also be included in the dry blend. Some of thematerials may be included as compounded pellets or powder produced byplastic compounds. A preferred dry blend is a mix comprising woodparticles mixed with mineral particles such as, for example, limestoneand this may be used to increase the water resistance. A higher amountof limestone may be used in the middle section than in the upper andlower sections of the board. The middle section may comprise wood/limestone fillers and the upper and lower sections may comprise onlywood fillers. The middle section may, for example, comprise a mix ofabout 40-60 wt %, e.g., 50 wt %, thermoplastic particles, 20-40 wt %,e.g., 30 wt %, limestone and 10-30 wt %, e.g., 20 wt %, wood particles.

Recycled fibre based materials such as, for example, particlescomprising recycled carpets, nylon fibres, packaging material or textilefibres, may also be used as fillers. Packaging materials comprisinglaminated layers of wood fibre material and thermoplastic material areespecially suitable as filler since they may comprise 10-20%thermoplastic material that may be used as a binder.

All the above-mentioned fillers may be used in some applications topartly or completely replace wood particles in separate layers of theboard or in the complete board.

The board 1′ may be cut and the edges may be formed by conventionalrotating tools. Scraping or carving may also be used preferably combinedwith increased temperature. Dust and chips from machining and scrapingmay be recycled and mixed into the dry blend.

FIG. 1b shows a panel 1 according to a preferred embodiment of theinvention where an upper surface layer 4 and a lower balancing layer 6comprising wood particles 20 and thermosetting resins are laminated to acore 5 comprising a WPP board 1′ as described above. Preferably, thesurface layer 4 and preferably also the balancing layer 6 are directlylaminated under heat and pressure in a separate hot—hot productionprocess to a pre-formed core 5. This two-step production method providesthe advantages that pressing parameters may be optimized to the formingof the board material that constitutes the core 5 and to the laminationof the surface and balancing layers to the core. Preferably the board iscooled to room temperature and stored for several days prior to thefinal lamination of the surface and balancing layers.

The core 5, the surface layer 4 and the balancing layer 6 may also beproduced in a one-step operation with a continuous or discontinuouspressing operation that preferably is a hot-cold process. A one stephot—hot process may also be used.

The thermosetting resin in the surface layer 4 and in the balancinglayer 6 may be melamine formaldehyde, urea formaldehyde or phenolformaldehyde resin, or a mix of these resins. The resins are preferablyapplied in dry powder form. Alternatively wood particles may beimpregnated with liquid resins and dried. Preferably thermoplasticparticles 21 a may be included in the surface layer 4 and the balancinglayer 6 in order to increase the bonding between the core 5 and thesurface layer 4 and to reduce moisture movements and shrinking forcesduring production and cooling to room temperature.

Preferably, the surface layer 4 comprises a decorative wear layer 3 thatis scattered on the WPP core. The decorative wear layer 3 may comprise,for example, a 300-500 g/m2, e.g., 400 g/m2, powder mix of 25-45 wt %,e.g., 36 wt %, melamine formaldehyde particles, 20-40 wt %, e.g., 30 wt%, PVC powder, 15-35 wt %, e.g., 25 wt %, wood fibres, 4-10 wt %, e.g.,7 wt %, Aluminium oxide 22 and 1-3 wt %, e.g., 2 wt %, colour pigments.The PVC powder may be a raw PVC powder without any additives such as,for example, emulsion PVC (Vestolit 6507) with a particle size of0.1-0.2 mm or emulsion PVC (Vestolit P 1353K) with a particle size ofabout 0.04-0.06 mm. A small particle size gives generally a higherquality since the small PVC particles gives a more homogenous materialstructure. It is preferred to use larger thermoplastic particles in thecore 5 than in the surface layer 4. Preferably, the thermoplasticparticles that are used to form the core 5 have an average size, whichis larger than about 0.10 mm, and the thermoplastic particles that areused to form the surface layer have an average size, which is smallerthan about 0.10 mm.

The dry blend that is used to form the balancing layer that preferablyalso is scattered on the WPP board comprises in this preferredembodiment about 100-300 g/m2, e.g., 200 g/m2, powder comprising a mixof 25-45 wt %, e.g., 35 wt %, melamine, 25-45 wt %, e.g., 35 wt %, woodfibres and 20-40 wt %, e.g., 30 wt %. PVC powder.

The powder layers are preferably applied in two steps. The balancinglayer 6 is in a first step scattered on the WPP core 5 and stabilized byspraying with water such that the melamine powder melts. The wet powdersurface is heated with IR light such that a hard but uncured powdersurface is formed allowing the WPP board to be turned with the scatteredsurface pointing downwards. The wear resistant decorative layer 3 isthereafter scattered on the upper part of the WPP core 5 and thedecorative powder layer 3 is stabilized in the same way as the balancinglayer 6.

The powder layers 3, 6 are thereafter directly laminated to the WPP corein a hot-hot pressing operation where, for example, a 30-50 bar, e.g.,40 bar pressure may be applied during 10-30 seconds, e.g., 20 seconds,under a temperature of 130-200° C., e.g., 160° C. Preferably the curedsurface layer 4 and the cured balancing layer 6 comprises individualthermoplastic particles 21 a spaced from each other and surrounded bywood particles 20 bonded to each other by the thermosetting resin. Thesurface layer 4 and the balancing layer 6 are preferably bonded to thecore 5 by cured thermosetting resins and preferably also by meltedthermoplastic particles 21 that are fused together when cooled to roomtemperature after heating under pressure.

The direct lamination pressure may be decreased considerably even in thecase when deep embossed surfaces are formed since the thermoplastic corebecomes softer when heated during the lamination and creates a more evencounter pressure than, for example, HDF. Embossed structures with adepth of about 0.5 mm and more may be formed with a direct laminationpressure of about 20 bar and even lower.

The layers have been modified considerably compared to known technologyin order to allow a direct lamination of a surface layer 4 comprisingthermosetting resins to a core 5 comprising thermoplastic material. Themodifications have been made based on the following principles.

-   -   The moisture movements of the WPP core material have been        increased considerably compared to a conventional WPC board in        order to be able to handle the moisture movements of the surface        layer that comprises a thermosetting resin. This has been        accomplished with a core that is based on a dry blend of wood        fibres and thermoplastic powder and that after pressing obtains        material properties allowing moisture to penetrate into the core        in high humidity and to evaporate in dry conditions.    -   Preferably, the moisture movement and the shrinking during        pressing and cooling to room temperatures of the powder based        surface layer comprising thermosetting resins and wood fibres        have been reduced and flexibility of the layer has been        increased by for example including thermoplastic particles into        the powder layer. Thermosetting resins in powder form and        thermoplastic particles in powder may be combined if they are        mixed with wood fibres that bond to both plastic types.    -   The WPP core is adapted such that a surface layer may be        directly laminated to the core with a hot-hot process. Forming a        core from a dry blend of small wood particles in powder form and        thermoplastic particles in powder form creates a matrix material        when pressed together with heat that comprises a considerable        amount of free uncoated fibres at surface portions of the core        that may be used to provide a strong bonding of a surface layer        comprising thermosetting resins to a core comprising        thermoplastic material.

One disadvantage may be that the upper part of the exposed cured surfacelayer comprises small thermoplastic particles 21 a that may be morestain sensitive than the majority of the exposed surface that comprisescured wood fibre/melamine material. The small melted thermoplasticparticles 21 a may also form small protrusions above the main surfacewhen the hot press table is opened after the lamination. Such smallprotrusions may be removed with a brushing operation when the surface iscooled down.

FIG. 1c shows that as an alternative, the thermoplastic particles 21 ain the decorative surface layer 3 may be covered with a transparent wearlayer 2 without thermoplastic particles. Such a transparent wear layer 2may be a powder overlay, preferably comprising melamine formaldehyderesins and cellulose fibres, preferably bleached cellulose fibres 20 c.The powder overlay may also comprise colour pigments and/or aluminiumoxide particles 22. A powder overlay mix may, for example, comprise60-85 wt %, e.g., 72.5 wt %, melamine formaldehyde resins, 50-30 wt %,e.g., 22.5 wt %, transparent cellulose fibres, and 2-8 wt %, e.g., 5 wt%, aluminium oxide particles. Such a powder overlay 2 may be very thin,for example, about 50-100 g/m2 and will not have any major effect on theshrinking forces during pressing and the moisture movement. The stainresistance will be superior to most thermoplastic surfaces. It is ofcourse possible to include small thermoplastic particles in the overlayand the stain resistance will be marginally affected especially if theparticles are small and the content is low. The transparent overlay 2may also be applied directly on the core 5. Alternatively, thetransparent wear layer 2 may be an overlay paper. A UV lacquer may alsobe used as protective layer.

Preferably, different fibre types and particles sizes may be used in thedifferent layers. The average size of the fibres or wood particles 20 ain the core 5 is preferably larger than the average size of the fibres20 b in the decorative wear layer 3. The transparent wear layer 2comprises preferably bleached cellulose fibres 20 c and the core 5 andthe decorative wear layer 3 comprises preferably opaque fibres 20 a, 20b. The content of the plastic particles in the core 5 may vary. A highercontent of thermoplastic particles may be used, for example, in themiddle parts of the core 5 than in the outer parts of the core and thefibre content in the outer parts of the core may be higher than in themiddle parts of the core. This will facilitate a strong bonding of thesurface layer 4 and the balancing layer 6 during direct lamination whenthermosetting resins float and penetrate into wood fibres of the woodparticles in the outer parts of the core 5.

A floor panel according to an embodiment of the invention is preferablycharacterized in that the content of the thermoplastic material ishigher in the core 5 than in the decorative wear layer 3. The content ofthe thermoplastic particles in the surface layer 4 may also vary and maybe higher in a lower layer closer to the core 5 than in an upper layercloser to the uppermost surface portion.

A surface layer 4 that comprises only thermosetting resins and woodfibres but no thermoplastic particles may also be direct laminated to aWPP core. The moisture movement of the surface layer 4 and the core 5may be adapted such that delamination is prevented. For example, thesurface layer may comprise urea formaldehyde resin that is less moisturesensitive than melamine formaldehyde resins.

FIG. 1d shows a floor panel 1 that may have the same surface layer 4 andbalancing layer 6 as described above and shown in FIGS. 1b and 1 c. Thesurface layer 4 may also comprise a digital print 19 applied on thepowder based decorative layer 4 preferably comprising a base colour. Thedigital print 19 may also be applied on a paper layer 18, comprising abase colour and incorporated into the decorative powder layer. The paperlayer may also be a printed paper impregnated with a melamine resin.Alternatively the paper 18 is not impregnated and is located betweenupper and lower powder layer comprising a thermosetting resin.Impregnation and bonding takes place automatically during pressing whenliquid melamine resins from the lower decorative powder layer 3 and theupper powder overlay 2 penetrate into the fibres of the paper. Powderoverlay 2 may be replaced by a conventional melamine impregnatedoverlay. PU coating and/or transparent plastic foils may also be used asan upper protective layer.

The core 5 comprises an upper 5 a and a lower part 5 c that is a WPPmaterial as described above and one or several intermediate layers 5 b.The intermediate layer is preferably a Mineral Plastic Particle (MPP)layer that comprises a mix of thermoplastic material and mineralfillers, for example, limestone. Such a layer may be rather elastic andsoft, especially if plasticizers are used and may provide sound dampingand softness to the floor. The intermediate layer is preferablyscattered as a dry blend, comprising thermoplastic particles in powderform and minerals in powder form, when the core 5 is formed in the sameway as the upper 5 a and the lower 5 c layers. The intermediate layermay also be produced in a separate step according to conventionalmethods used to form the known LVT layers and may be bonded to thescattered layers with heat and pressure. The intermediate layer andother core layers may also comprise a mix of minerals, wood fibres andthermoplastic particles. Chalk, talcum powder, limestone and corkparticles may, for example, be used as fillers. Very thin layers, forexample, 0.1-0.5 mm or 0.2-1.0 mm may be sufficient to improve the soundproperties and the flexibility of the panel.

A wide range of recycled wood and mineral particles may be used in allcore layers and such materials may be mixed together. It is preferred touse HDF waste or particleboard waste or similar wood based waste thatcomprises binders and that is not suitable for burning due toenvironmental considerations. Old replaced laminate floors may be milledto small particles and used as filler.

The WPP board may be replaced with other core materials, for example,HDF boards and mineral boards, in all embodiments of the invention. Amix of wood fibres, thermosetting particles, thermoplastic particles,pigments and wear resistant particles with or without a powder overlayas described above may be used as a surface layer. This provides severaladvantages compared to conventional technology. The shrinking duringproduction and moisture movements will be reduced. Thinner balancinglayer may be used. It is also cost efficient to replace, for example,melamine with thermoplastic particles.

FIG. 2a shows plastic compounds 23 in pellet form that have beenproduced, for example, by an extruder. The plastic compound 23 compriseswood particles 20 that have been mixed with a thermoplastic material 21under heat and pressure to a homogenous mix. Such thermoplasticcompounds 23 are used to form a Wood Plastic Composite board or aso-called WPC board 40 as shown in FIG. 2 b. The board 40 and its upper11 and lower 13 parts comprise a homogenous mix of wood particles 20 andthermoplastic material 21 and practically all wood particles areembedded in the thermoplastic material.

FIG. 2c shows a dry blend 25 of thermoplastic particles 21 a in powderform and wood particles 20 in powder form. The average length L of woodparticles 20 is preferably about 1.0 mm or smaller. The thermoplasticparticles 21 a are preferably even smaller and may have a size of about0.1-0.2 mm. FIG. 2d shows that a Wood Plastic Particle board 41 or a WPPboard is formed when the dry blend 25 is pressed together under heat.The dry blend 25 forms a material structure that is not completelyhomogenous and this allows the wood fibres of the wood particles 20 toshrink and swell slightly in different humidity. There may be asufficient amount of wood fibres in the upper 11 and lower 13 surfaceparts of the WPP board that are not completely covered by thermoplasticmaterial 21 and this provides a strong bonding between the WPP board anda surface layer when heat and pressure is applied to a surface layerthat may comprise thermosetting particles or thermoplastic andthermosetting particles mixed with wood fibres. The wood particles 20 inthe upper part 11 of the board 41 and preferably also at the lower part13 of the board may protrude slightly above or below the meltedthermoplastic material 21 when the pressure from the forming of theboard is released.

FIGS. 2e-2h show a similar forming of a layer or a board material asdiscussed above with reference to FIGS. 2a -d. In this preferredembodiment wood particles 20 have been replaced with mineral particles24. A Mineral Plastic Composite (MPC) board 42 is formed when a plasticcompound 23 comprising mineral particles 24 is used as raw material asdisclosed in FIG. 2e and FIG. 2 f. A Mineral Plastic Particle board 43is formed when a dry blend 25 comprising mineral particles 24 is used asraw material as disclosed in FIG. 2g and FIG. 2 h.

FIG. 3a shows a production equipment and a production method that may beused to form a panel from a dry blend 25 of thermoplastic particles andfillers such as wood particles or mineral particles according to theembodiments described above. A dry blend 25 is applied by a scatteringunit 33 on a lower belt 31, compressed through a 2-roll calender 34 andheated in a pre-heating zone 35. The heated dry blend is thereafterpressed between the lower belt 31 and the upper belt 32 under hightemperature in a pressing and heating zone 36 such that thethermoplastic particles melt. The melted powder is thereafter sized tothe required thickness by nip rollers 37 and cooled under pressure in apressing and cooling zone 38 to the required outlet temperature suchthat, for example, a wood plastic particle board 41 or a mineral plasticparticle board 43 is formed. The dry blend may comprise a mix ofdifferent fillers, for example, wood particles and mineral particles.

The wood 41 or mineral 43 particle board may comprise one or severalindividually produced layer as described above that may be fusedtogether with heat and pressure to a thicker multi-layer board.Preferably, the individual layers may have a thickness of about 1-3 mm.

A board may be formed with a very low pressure, for example, a few bar.Improved quality may be obtained with higher pressures, for example,5-10 bar.

The production may comprise two continuous presses. One press may be ahot press and one press may be a cold press. The advantage is that it isnot necessary to heat and cool the upper 32 and lower 31 belts.

A WPP board may also be produced with a hot-hot pressing operation incontinuous or discontinuous presses. A cooling without pressureincreases the porosity of the board. Internal bonding strength and waterresistance will generally be lower than for similar boards produced witha hot-cold pressing operation. This may be compensated with a highercontent of thermoplastic particles.

FIG. 3b shows an extruder and a production method to form a layercomprising thermoplastic particles and fillers. A hopper 27 is filledwith thermoplastic particles 21 a and fillers, for example, mineralfillers 24 or wood fillers 20 and the particles are mixed according tospecifications and loaded into the feed section of the extruder. Thematerial is fed through one or two screws, melted and mixed into ahomogenous blend by a combination of heat and shearing forces. From thescrew tip, the material is filtered and pressed through a flat sheet die29 that provides a paste with essentially the same width and thicknessas the finished layer. The die 29 has generally oiltemperature-controlled adjustable die lips.

The melted paste is further processed with 3-roll polishing calenders 30comprising a fixed central hot roll and two adjustable rolls on eachside for final calibration of a layer that may be used as a core layer 5a, 5 b, 5 c or a surface layer 4 in a floor panel according to theinvention. A board may comprise several layers, which are formed as WPP,WPC and MPC board material as described above.

The extruder may be used to produce plastic compounds in pellet form.

FIG. 3c shows that separate layers, for example, a core layer 5, dryblends 25 and decorative foils 17 may be formed and laminated togetherin a continuous hot-cold pressing operation. The pressing stepsdescribed in FIGS. 3a and 3c may be combined in a continuous productionline with cooling and annealing downstream equipment.

FIG. 3d shows a discontinuous press 44 that may be used to apply asurface layer 3 and a balancing layer 6 comprising thermosetting resins,wood particles and preferably thermoplastic particles to a core 5comprising thermoplastic particles and fillers, preferably wood fillers.The application is preferably made with a hot-hot pressing method.

FIGS. 4a and 4b show a floor panel 1 with a locking system comprising atongue 10 and a groove 9 for vertical locking and a protruding strip 7with a locking element 8 in a first edge 1 b that cooperates with alocking groove 14 in a second adjacent edge 1 a and locks the edges in ahorizontal direction. The tongue 10 may be formed on the second edgecomprising a locking groove, as shown in FIG. 4 a. The tongue 10 mayalso be formed on the first edge comprising the strip 7 (not shown). Thetongue groove 9 may also be formed on the first or the second edge. Thepanel has a core 5 with an upper layer 5 a and a lower layer 5 c, eachcomprising wood particles mixed with thermoplastic material and anintermediate layer 5 b preferably comprising mineral particles andthermoplastic material. The intermediate layer 5 b is preferably softerthan the upper 5 a and lower 5 c layers and may be used to provide sounddamping. The strip 7, the locking element 8 and the locking groove 14are formed in the rigid lower layer 5 c.

The intermediate layer 5 b may be formed as a separate layer and may belaminated between the upper and lower layers that may be applied as adry blend prior to pressing. The intermediate layer 5 b may also beapplied as a dry blend as shown in FIG. 4 b.

FIG. 4c shows an embodiment where the intermediate layer 5 b comprises ahigher content of thermoplastic material and a lower fibre content thanan upper 5 a and a lower 5 c layer. Such a core may be formed with, forexample, increased rigidity and moisture resistance and this could becombined with a direct lamination of the surface layer 4 and thebalancing layer 6 that is made against layers that comprise a higherfibre content.

FIG. 4d shows an embodiment where a softer layer 5 b, preferablycomprising thermoplastic material mixed with wood fibres and/or mineralsis applied at an upper part of the core 5, preferably above the tongue10 and the groove 9. Such a core provides increased sound damping.

A lot of separate layers and materials may be included in the core, forexample, glass fibres or metal foils. Foils, for example, aluminiumfoils, may improve heat distribution when floor heating is used in thesub floor. Foils that provide heat may also be incorporated into thecore.

EXAMPLE

48 wt % of wood particles and dry fibres from spruce, with a moisturecontent of about 1% and sieved trough a 400 microns mesh, was mixed with48 wt % of PVC particles (Vestolit 1353) with an average size 60 micronsand with 4 wt % of grey colour pigments such that a dry blend of woodparticles in powder form and thermoplastic particles in powder form wasobtained. The dry blend was thereafter applied as a 12 mm thick powderlayer on a 0.6 mm aluminium sheet, pre-heated in a hot air oven to 100°C. and brought into a hydraulic press. The dry blend was pressed during2 minutes under a heat of 160° C. and with a pressure of 10 bar andcooled. The press was opened when the temperature reached to 80° C. Aflat and rigid 4 mm grey Wood Plastic Particle (WPP) board was obtainedat room temperature.

A balancing layer of 200 g/m2 of powder comprising a mix of 35 wt %melamine, 35 wt % wood fibres and 30 wt % PVC powder (Vestolit 1353) wasscattered on the WPP board, spayed with water and dried with an IR lampsuch that the mix was bonded to the WPP board allowing the board to beturned upside down with the dry uncured balancing layer pointingdownwards.

A 400 g/m2 decorative and wear resistant surface layer with a lightbrown base colour was thereafter scattered on the upper side of the WPPcore. The decorative wear layer comprised a powder mix of 36 wt %melamine formaldehyde particles, 30 wt % raw PVC powder (Vestolit P1353K) with a particle size of about 0.04-0.06 mm, 25 wt % recycled woodfibres/wood particles, 7 wt % Aluminium oxide and 2 wt % colourpigments.

A water based ink comprising 30% water based pigment dispersions and 70%glycol, water and acrylic binder was injected with a digital Piezo inkhead into the powder and a wood grain decor was formed.

A 50 g/m2 powder overlay mix comprising 72.5 wt % melamine resins, 22.5wt % bleached cellulose fibres, and 5 wt % aluminium oxide particles wasscattered over the digital print. The powder overlay was sprayed withwater based micro drops comprising a release agent and dried with IRlamp in order to prevent a “blow out” of powder during the pressingoperation.

The WPP board with the decorative surface and the balancing layer waspressed in a conventional direct lamination hot—hot press under a heatof 160° C. and with 20 bar pressure during 20 seconds to a floor blankwith a water proof core comprising thermoplastic material and a stainand wear resistant digitally printed surface layer comprising athermosetting resin.

The floor blank was after cooling to room temperature cut intoindividual floor panels and the edges were formed with an anglinglocking system on the long edges and a fold down system on the shortedges comprising a separate flexible tongue that allows locking withvertical displacement, angling and horizontal snapping.

1. A building panel comprising a core and a surface layer, wherein thecore comprises a mix of thermoplastic material and wood particles,wherein the surface layer comprises a composite material comprising woodparticles bonded to each other by a cured thermosetting resin, andwherein the surface layer is bonded to the core by the curedthermosetting resin.
 2. A building panel as claimed in claim 1, whereinthe thermosetting resin is an amino resin.
 3. A building panel asclaimed in claim 1, wherein the surface layer further comprisesthermoplastic particles.
 4. A building panel as claimed in claim 1,wherein the thermoplastic material of the core comprises polyvinylchloride, polypropylene, or polyethylene.
 5. A building panel as claimedin claim 1, wherein the surface layer comprises thermoplastic particleswith a size of about 0.2 mm or less.
 6. A building panel as claimed inclaim 1, wherein the wood particles of the surface layer have an averagesize that is smaller than an average size of the wood particles of thecore.
 7. A building panel as claimed in claim 1, wherein the buildingpanel is a floor panel.
 8. A building panel as claimed in claim 1,wherein one pair of opposite edges is formed with a mechanical lockingsystem comprising a protruding strip and wherein the protruding stripcomprises wood particles and thermoplastic material.
 9. A boardcomprising an upper surface, a lower surface, and an intermediateportion located between the upper surface and the lower surface, whereinthe intermediate portion comprises wood particles embedded in athermoplastic material, and wherein the upper surface comprisesessentially raw wood particles adapted to bond to a thermosetting resin.10. A board as claimed in claim 9, wherein the essentially raw woodparticles protrude beyond the upper surface.
 11. A board as claimed inclaim 9, wherein the thermoplastic material comprises polyvinylchloride, polypropylene, or polyethylene.
 12. A board as claimed inclaim 9, wherein the average size of the essentially raw wood particlesat the upper surface is about 0.1-1.0 mm.
 13. A building panel asclaimed in claim 1, wherein the surface layer comprises two layersarranged above each other, and wherein an upper layer of said two layerscomprises bleached cellulose fibres and thermosetting particles.
 14. Abuilding panel as claimed in claim 1, wherein the content of the woodparticles in the core is in the range of about 30-80 wt %.
 15. Abuilding panel as claimed in claim 3, wherein the thermoplastic materialis comprised in the powder mix of the surface layer and comprisesthermoplastic particles, wherein the thermoplastic particles have anaverage size of about 0.2 mm or less.
 16. A building panel as claimed inclaim 1, wherein the core comprises an intermediate layer and an upperlayer, and wherein the intermediate layer comprises a higher amount ofthermoplastic material than the upper layer.
 17. A building panel asclaimed in claim 16, wherein the intermediate layer further compriseslimestone particles.
 18. A building panel as claimed in claim 1, whereinthe surface layer comprises a transparent wear layer comprising athermosetting resin, wear resistant particles, and cellulose fibres. 19.A building panel as claimed in claim 18, wherein the thermosetting resinof the transparent wear layer is an amino resin.
 20. A board as claimedin claim 9, wherein the essentially raw wood particles protrude beyondthe lower surface.